1. Field of the Invention
The invention relates to a device for locking the movable counter gripping head of a sheet or section stretcher, comprising pressure elements which are provided between the counter gripping head and the stretching gripping head and which are disposed on both sides and at the level of the material to be stretched.
2. Description of the Prior Art
In sheet or section stretchers the sheet to be stretched is gripped in gripping heads which are moved away from one another by hydraulic cylinder units. Owing to the varying lengths of sheet one of the gripping heads is movable and may be locked in various positions in a positive manner with the force-transmitting elements, which may be the machine framework or special press or tie rods. Locking is effected in any position by way of nuts and spindles or in stepwise positions by way of bayonet sockets which engage in spindles constructed in a bayonet shape. The spindles are the pressure elements, which are prevented from buckling in a trouble-free manner only where the material to be stretched is short.
Solutions are also known in which the counter gripping head is locked in the pressure elements by means of pins and holes. In this case the spacing from hole to hole is relatively large, which results in increased strokes of the cylinder units. In order to reduce the effective spacing, the counter gripping head is also provided with two pins which are arranged one behind the other at a distance of 1.5.times.hole spacing and which may be inserted alternately, so that an effective spacing of 0.5.times.hole spacing is produced. On the one hand the effective spacing from position to position is still relatively large, even in the case of the latter solution, and on the other hand the device necessary for inserting and extracting the pins projects far above the level of the other components of the apparatus.